1. Field of the Invention
The present invention relates to a dynamic pressure gas bearing and, more particularly, to a dynamic pressure gas bearing for use in main spindles, for example, of a polygon mirror scanner.
2. Description of the Prior Art
Referring to FIG. 6, a prior art dynamic pressure gas bearing for use in a polygon mirror scanner is shown. Reference numeral 12 is a base in which the center portion thereof is provided with a shaft 13. Around the shaft 13, a cylindrical sleeve 14 having a flanged portion 14a is rotatably mounted with a suitable clearance between the shaft 13 and the cylindrical sleeve 14. The side wall surface of the shaft 13 is divided into three portions by two concentric circles. Each of divided portions is provided with a plurality of grooves in which grooves 13a are in a herringbone pattern, grooves 13c in a helical pattern and grooves 13b in a herringbone pattern, for generating a dynamic air pressure. The base 12 is further provided with a stator 15 thereon. The sleeve 14 is provided with a rotor 16 there around. The stator 15 and the rotor 16 construct a motor 17. The sleeve 14 is further provided with a round lid 18 inserted in one end positioned far from the base 13. The round lid 18 has a nozzle hole 18a passing through in alignment with the axis. A polygon mirror 19 having a center hole is provided around the sleeve 14 so as to sit on the flange part 14b. On the polygon mirror 19, a fixing plate having a center hole is provided around the sleeve 13. The sleeve 14, the polygon mirror 19, and the fixing plate 20 are integrally connected by bolts. The base 12 is provided with a cylindrical cover 21 to construct a housing for accommodating the above described matters therein and for keeping dust trespassing in. In the side wall of the cylindrical cover 21, a opening covered with a shield glass 21a is provided.
In operation, when the power is supplied to the dynamic pressure gas bearing described in the above, the motor 17 drives the sleeve 14. As the sleeve 14 rotates, the dynamic pressure generating grooves 13a and 13b generate a pumping effect by which the sleeve 14 will be rotated in no contact with the shaft 13. The round lid 18 is in contact with the shaft 13 when the sleeve 14 does not rotate. During the rotation, the air pumped by the helically patterned grooves 13c will generate a thrust force to lift us the round lid in a vertical direction shown by an arrow "A", when the pumped air blows out through the nozzle hole 8a. Thus, the round lid 18 lifts up, loses a contact with the shaft 13 and hovers in the air. The laser beam D enters into the housing through the shield glass 21a and reaches the polygon mirror 19 by which the laser beam is reflected.
However, a conventional dynamic pressure gas bearing as described above, shown in FIG. 6, may has such a problem as a seizure of the bearing shaft 2 caused by the dust entered between the shaft 13 and the sleeve 14. This dust is coming from, for example, the insulative coat of coils for the stator 15 and is fine enough to be measured in some micro meters order.